1. Field
The invention relates to circuits which generate random bit sequences.
2. Background Information
Random bit sequences are sequences of binary signals lacking discernible patterns or repetition over time. Random bit sequences are useful in encryption operations and other electrical operations which employ unpredictable signal sequences. A xe2x80x9ctruexe2x80x9d random sequence may be derived from thermal noise characteristics of electronic components. FIG. 5 shows a prior art circuit 500 which employs thermal noise characteristics of electrical components to generate a random bit sequence. A resistor 502 and capacitor 504 pair provides a voltage signal to the positive input terminal of a differential amplifier 510. Another resistor 506 and capacitor 508 pair provide a voltage signal to the negative terminal of the differential amplifier 510. The differential voltage provided to the terminals of the differential amplifier 510 will vary randomly due to the thermal noise of the resistors. The voltage signal output by the differential amplifier 510 amplifies these random variations. For example, differential amplifier 510 may amplify the voltage between the positive and negative input terminals by a factor of 200 or more. The amplified differential voltage is input to a voltage controlled oscillator 512 (VCO). VCO 512 provides a periodic signal with a frequency that varies according to the voltage input from the differential amplifier 510. The frequency of the VCO signal thus varies according to the random variations in the voltage signal provided to the VCO 512. Circuit 514 converts the VCO signal into a ground-referenced signal which is supplied to the clock terminal of a D-style flip-flop 518. Of course, a differential sampler circuit could be used in place of the conventional flip-flop, in which case the VCO signal could be applied to the sampler without converting to a ground-based signal with circuit 514. Flip-flop 518 (or other sampler circuit) may be configured to propagate the signal at its D input terminal (the D signal) to its Q output terminal upon receiving an edge of VCO signal (this is often referred to as xe2x80x9clatchingxe2x80x9d the D signal). The random variations in the frequency of VCO signal result in random variations in the timing of the edges of VCO signal. Therefore, flip-flop 518 latches the D signal at randomly-varying points in time. High frequency oscillator 516 (HFO) provides the D signal to the flip-flop 518. D signal produced by HFO 516 transitions from logical low to logical high rapidly and periodically; flip-flop 518 will latch the D signal at logical low or at logical high randomly, according to the random variations in the timing of VCO signal edges. The latched D signal is output on terminal Q as random bit sequence 520.
A disadvantage of circuit 500 is that amplifier 510 comprises numerous analog components. For example, the resistors and capacitors and the differential amplifier 510 may be implemented as analog circuits. Analog component designs tend to not scale well to higher frequency and lower voltage processes. Furthermore, the internal analog components of amplifier 510 may accumulate signals over time (such as so-called xe2x80x9c1/f noisexe2x80x9d) which result in deviations in the amplifier""s desired performance. A further disadvantage of circuit 500 is that in order to maintain the randomness of the variation in the VCO output signal, differential amplifier 510 must be prevented from xe2x80x9crailingxe2x80x9d. Railing occurs when the input signals to the amplifier or the deviations resulting from xe2x80x9c1/f noisexe2x80x9d cause the amplifier output signal to reach a maximum predetermined low or high voltage. In other words, amplifier output signal may level off at a low or high DC voltage level. Once this happens, the VCO output signal frequency becomes more or less constant. The VCO output signal no longer reflects random variations in frequency. Consequently, the bit stream 512 provided at output terminal Q of flip-flop 518 will lose its random characteristics. Preventing the output of differential amplifier 520 from railing involves complex equalization circuitry. Such circuitry results in increased circuit cost, circuit size, and design time.
A circuit includes a first oscillator having transistors to produce a first signal with random variations resulting from device channel resistance of the transistors.